Pumps and pumping devices are in wide use in industry. For example, one type of pump used in industries such as, for example, automotive paints, paper coatings, and other industrial processes is a positive displacement pump. One type of positive displacement pump is a rotary two-rotor pump in the form of an external circumferential piston pump.
An exemplary pump of this type includes a body defining a chamber having an inlet and outlet. Inside the chamber are disposed two counter rotating rotors. The rotors are driven by a motor and gear box to force the material from the inlet to the outlet.
An example of such a pump is depicted in FIG. 4. The pump 1 includes a first rotor 10 and a second rotor 12 disposed in a chamber defined by a housing body 14 which serves as a housing in combination with an end plate 15. The first rotor 10 is driven by a drive shaft 16 and a fastening nut 17 on the shaft end, and the second rotor 12 is driven by a shaft not visible in FIG. 4 and held by a similar nut 17. FIG. 4 also depicts an inlet 20 and outlet 22.
A body hub clearance gap 24 exists between the rotating rotor 10 and a stationary projection referred to as a body hub 26 that is part of the housing body 14. The rotors 10 and 12 are driven by a gear box 28.
FIG. 5 is an external view of the pump 1, indicating the section line through which the sectional views in the other drawing figures are taken.
Turning to the section view of FIG. 6, the prior art pump 1 is further illustrated. In particular, in this figure it can be seen that a seal chamber 30 is defined between an inner face 34 of the body hub 32 and an outer face 36 of the rotor 10 which is a central mounting shaft portion of the rotor 10.
The dimensions of the body hub clearance gap 24 in the prior art are important to volumetric efficiency and pump performance. This is due to a relatively small sealing area that exists at the body hub clearance gap 24 and also to the location of the clearance gap 24 in the pumping path between the inlet 20 and outlet 22.
The body hub 32 and the rotor 10 have surfaces that form the body hub clearance gap 24 which are subject to high fluid velocity that sometimes results in rapid wear, especially when the pumped material contains abrasive particles. In certain applications such as, for example, automotive paint and paper coatings, the abrasive wear can dramatically reduce the useful service life of the pump.
The body hub clearance gap 24 is a location of sliding frictional contact, or near-contact, between the end tip 25 of the body hub 32 and an exposed axial face of the rotor 10. This sliding contact, or near-contact, accomplishes an imperfect “seal” of the contact area. This “seal” is subject to wear over time.
Referring to further to FIG. 6, it can be noted that during operation, while the pump is moving fluid from the inlet 20 to the outlet 22, the pressure of the fluid on the outlet side 22 is raised so that the pressure is higher at the outlet side 22 than the inlet side 20. This pressure differential (between the outlet side 22 past the rotors 10 and 12, compared to the inlet side 20 before the rotors 10 and 12), causes the pumped fluid to tend to leak back towards the inlet side 20 through the body hub clearance gap 24.
Although the body hub clearance gap 24 is actually a toroidal ring in its overall shape, the section view of FIG. 6 illustrates that the body hub clearance gap 24 can be thought of two body hub clearance gaps, one gap indicated at 24 and the other gap indicated at 25. The pumped fluid thus can be thought of as having two successive leak paths which the fluid can be thought of as leaking through in a serial fashion.
It would be desirable to reduce one or both of these leak paths at each rotor if possible. Accordingly, it would be desirable to reduce the pressure on the seal, referred to as a seal pressure, so that less material leaks through the clearance gap 24, and so that in the case of abrasive materials, reduction of the wear or erosion of the components in the area of the clearance gap 24 would occur. Wear in this area is undesirable because it reduces pump efficiency over the long term.
Accordingly, it is desirable to provide a method and apparatus that can yield improved performance and/or wear characteristics in a circumferential piston pump.